Utility vehicles and other off road machines may have an MFWD to provide power to the front wheels if the rear wheels slip and lose traction. With the operator controlled 4WD switch in the on position, roller clutches in the front axle gear case will engage and provide drive to the front wheels if rear wheel speed is faster than front wheel speed. With no speed differential between the front and rear wheels, the front drive does not engage and the vehicle is driven by the rear wheels only.
With the 4WD switch in the on position and the transmission in either forward or reverse, voltage may be supplied to a roller cage drag mechanism such as a solenoid. The voltage is stopped when the machine is shifted into neutral. For example, if a solenoid is energized, a plunger may be extended into the path of the rotating tabs of an actuating washer, stopping the outer washer. Wave washers between the inner and outer washers place a drag between the outer (stopped) washer and the inner washer. The inner washer is keyed to a roller brake assembly which is connected to a roller cage by tabs.
The roller cage lies inside a ring gear hub. Each side of the roller cage contains cylindrical rollers that roll around the output hub to each axle. The rollers may be generally cylindrical or may have other geometries.
The drag imposed on the roller cage through the wave washers and tabs forces the rollers slightly off center in the openings of the roller cage. When the rear tires slip and spin more than about 15% faster than the front tires, the rollers move against ramps in the rotating ring gear hub and are forced inward. The inward movement presses the rollers against the left and right output hubs. At this point, all components begin rotating together.
Alternatively, the roller cage drag mechanism may be an electromagnet instead of a solenoid. The 4WD switch may energize an electromagnet that imposes drag on an armature plate, and through direct contact with the plate, to the roller cage.
When the vehicle stops and the operator shifts the transmission from forward to reverse, the voltage to the roller cage drag mechanism is momentarily stopped (neutral switch opened). This allows the springs on each roller to rotate the roller cage and rollers back into a neutral position.
However, certain driving maneuvers may allow the rollers to become wedged tightly between the ramps in the ring gear and output hub, and not move back to the neutral position. For example, in a reverse panic stop, an operator may drive the vehicle in reverse and then apply the brake suddenly to lock the front wheels and tires. High forces tend to push the rollers further up the ramps where they may become wedged against the output hub. Rollers also may become wedged by driving the vehicle up a sloped surface in forward or reverse, then allowing it to roll back down without applying the brakes or shifting the transmission out of forward or reverse.
If a vehicle is driven with wedged rollers, the vehicle drive train may be damaged by high sustained torque transferred through the driveshaft. For example, driveshaft components including the CV joint may be damaged. A mechanical front wheel drive roller wedging control system is needed to prevent wedging of rollers between the ramps in the ring gear and the output hub.